Welcome to the Magix Sound Forge users forum @PETER-CHMIELOWSKI.

Is the MC codec grayed out or totally missing? As a work around, you could use the Sony AVC/AAC <.mp4> codec which is on par with the MC AAC codec. Alternately there are the Nero AAC and Fraunhofer FDK AAC encoders, which are allegedly better than the MC or Sony (though my I cannot detect an audible difference). The non-native encoders are command line based and need a PCM intermediary file.

Look for a "mp4plug2" folder in Program files> Sound Forge AS "FileIO Plug-Ins" folder, if missing, it can be copy-pasted from another version of SF or Vegas. Otherwise, you could use the Sony AVC/AAC <.mp4> codec.

You can also try a reinstall, or.. uninstall, reboot, then reinstall,